How to set OFF to User online mode by auto logout after form atuhendcation timeout

Hi,
The below method is working when user click the logout button an di am using form authendication with 20 minutes;
But i have got a problem, after the timeout when the page redirect to login page, i need to call the method, but i could not
get the page user identity name because the values are expires;
How to set user online  set to offline.
 
  void logout()
{
   MembershipUser user;         user = Membership.GetUser(Page.User.Identity.Name);
   user.LastActivityDate = DateTime.Now.AddMinutes(-20);
  Membership.UpdateUser(user);
  FormsAuthentication.SignOut();
    }
in web.config
<authentication mode="Forms">
       <forms loginUrl="~/Account/Log ...